The present invention generally relates to a video signal processing circuit of a video camera, and more particularly relates to a video signal processing circuit of a single chip colour camera which employs a solid state pickup device such as a charge coupled device (hereinafter referred to as a CCD).
An example of the conventional video signal processing circuit of the single chip colour camera is described with reference to FIG. 7. FIG.7 shows a system block diagram of a signal processing circuit 20 of the single chip colour camera which employs the CCD. Due to incident light from a camera lens (not shown) an electric charge is generated and accumulated in a CCD image sensor 12. The accumulated electric charge is read out from the CCD image sensor 12 responsive to a horizontal driving pulse .phi.H and a vertical driving pulse .phi.V. The read out accumulated charge is inputted to a sample and hold circuit 13 in a form of CCD output gate reset pulse .phi.R of approximately 0.6 volts p-p being superposed with a video signal of approximately 0.1 volt p-p. The sample and hold circuit 13 transforms the CCD output gate reset pulse .phi.R superimposed with the video signal into a continuous signal and supplies the continuous signal to an automatic gain control circuit (hereinafter referred to as an AGC circuit) 14. The AGC circuit 14 has a variable gain of 0 to 9 dB for example, and operates so that when an iris within the single chip colour camera is opened, the AGC circuit 14 adjusts the video signal input therein to a suitable level, and supplies the video signal subjected to the adjustment to low pass filters (hereinafter referred to as LPF) 15 and 16, and a band pass filter (hereinafter referred to as BPF) 17.
An explanation is now given of an operation of processing a luminance signal from the video signal. The LPF 15 has a cut-off frequency of approximately 3.2 MHz, and accordingly a luminance signal Y.sub.H is obtained by the LPF 15. In addition, a horizontal resolution of up to approximately 270 lines can be determined by the LPF 15. At a following stage, an equalizer 24 compensates a time difference between the luminance signal Y.sub.H and a colour signal which is separately processed from the video signal as described later and thus adds a time delay of approximately 0.4 .mu.sec to the luminance signal Y.sub.H. Next, a clamping circuit 25 determines a set-up value of the luminance signal, and thereafter a gamma compensation circuit 26 determines a .gamma. characteristic, that is a .gamma. value, for determining an output characteristic of the luminance signal. The .gamma. characteristic of the signal obtained from the CCD image sensor 12 is such that .gamma. is approximately equal to 1. However, the gamma compensation circuit 26 is designed to perform a compensation so as to obtain a gamma value of 0.45, and therefore compensates the luminance signal Y.sub.H output from the single chip colour camera to the gamma value of 0.45. Following the gamma compensation circuit 26, a trap circuit 31 removes a cross-talk component of a colour signal of 3.97 MHz from luminance signal Y.sub.H, and thereafter supplies the luminance signal Y.sub.H to a delay circuit 32 and an operational circuit 33. A horizontal contour compensation signal for compensating a horizontal contour of the luminance signal Y.sub.H is produced through the delay circuit 32 and a differential amplifier accommodated in the operational circuit 33. The operational circuit 33 adds the contour compensation signal to the luminance signal Y.sub.H and thereafter outputs the luminance signal Y.sub.H to an adder 37. Further, a vertical contour compensation signal is produced from a luminance signal Y.sub.L which is obtained through the LPF 16 which has a cut-off frequency at approximately 0.8 MHz. The luminance signal Y.sub.L is subjected to a similar process to that described above for the luminance signal Y.sub.H with respect to a clamping circuit 34, a gamma compensation circuit 27, a delay circuit 35, and an operational circuit 36. Thus, the vertical contour compensation signal which is added to the luminance signal Y.sub.L is outputted to the adder 37.
Next, a description is given of an operation of processing the colour signal in the conventional video signal processing circuit. The video signal is supplied from the AGC circuit 14 to the BPF 17 which has a band width of 3.97+1 MHz and thereby extracts a signal from the video signal. This extracted signal is supplied to and detected by a synchronous detector (hereinafter referred to as a detector) 18 and as a result, a colour difference signal is obtained. The colour difference signal has values of 2R-G and 2B-G, where R denotes a red primary colour signal, B denotes a blue primary colour signal and G denotes a green primary colour signal. The colour difference signal alternates between the values 2R-G and 2B-G for every one scanning line and is a line sequential colour difference signal. The line sequential colour difference signal is subjected to a band limitation to 0.8 MHz in a LPF 19. Thereafter the luminance signal Y.sub.L supplied from the LPF 16 is added to the line sequential colour difference signal in an adder 21. As a result, the green signal G is cancelled out from the line sequential colour difference signal having the values 2R-G and 2B-G, so that a line sequential colour signal having values of 2R and 2B is outputted from the adder 21. Next, white balance gain control amplifiers (hereinafter referred to as a white balance GCA) 22 and 23 determine a set-up value and a white balance of the primary colour signals R and B. Thereafter, the primary colour signals R and B are subjected to a gamma compensation in respective gamma compensation circuits 28 and 29 and then are supplied to respective colour difference signal circuits (subtracting circuit) 38 and 39. The luminance signal Y.sub.L from the gamma compensation circuit 27 is supplied to both of the colour difference signal circuits 38 and 39 so that line sequential colour difference signals R-Y.sub.L /B-Y.sub.L are produced therefrom and supplied to a sequential-to-simultaneous conversion circuit 49. The sequential-to-simultaneous conversion circuit comprises a switching circuit 41, a delay circuit 44 which delays the sequential colour difference signal by one horizontal scanning interval, and switching circuits 42 and 43. The line sequential colour difference signals R-Y.sub.L /B-Y.sub.L are supplied to the switching circuit 41 which switches for every one scanning line and thereby obtains a continuous line sequential colour difference signal. Thereafter, two line sequential colour difference signals R-Y.sub.L /B-Y.sub.L are formed such that one of the line sequential colour difference signals R-Y.sub.L /B-Y.sub.L is delayed by the delay circuit 44 by one horizontal scanning interval. Next, the switching circuits 42 and 43 perform switching operations so as to convert the two line sequential colour difference signals R-Y.sub.L /B-Y.sub.L into two colour difference signals R-Y.sub.L and B-Y.sub.L which occur simultaneously for each horizontal scanning interval of 1H. The colour difference signals R-Y.sub.L and B-Y.sub.L from the sequential-to-simultaneous conversion circuit 49 are supplied to an encoder 45 where a balanced modulation is carried out on the respective colour subcarriers with the colour difference signals R-Y.sub.L and B-Y.sub.L respectively and the balanced modulated signals are mixed together. Thus, a chroma signal is outputted from the encoder 45. In the case of the NTSC system for example, the chroma signal has a frequency of 3.58 MHz.
However, the conventional CCD single chip colour camera does not have a construction so as to compensate by electrical means an error in luminance composition, and instead performs the compensation for the error in luminance composition by an optical construction. That is, the error in luminance composition is compensated by mounting a colour temperature conversion filter (an optical filter) for compensating for a change in colour temperature in front of the camera lens.
In general, the colour camera is adjusted by taking a reference object which is illuminated with a predetermined colour temperature of 3200 K for example. However, when taking an arbitrary picked object which is illuminated by light of a different colour temperature it is necessary to adjust the colour camera in order to obtain the predetermined colour temperature. There is a method of performing the adjustment optically by mounting a colour temperature conversion filter in front of the camera lens so as to convert the different colour temperature to 3200 K. This method for adjusting the colour temperature with respect to the conventional circuit by using the optical filter is beneficial in that the error in luminance is reduced. However, the use of the optical filter results in an increased weight of the colour camera and is a hindrance to manipulate a colour camera that should be compact and light in weight.